This invention relates to apparatus for receiving an AM stereophonic signal and, more particularly, to such apparatus whereby the mode of operation thereof is changed over from a stereo mode, whereby stereophonic information is reproduced, to a mono mode, whereby monaural information is reproduced, if it is determined that the stereophonic information cannot be properly recovered.
It has been proposed to broadcast stereophonic information on conventional AM broadcast frequencies. Such AM stereophonic transmissions are intended to be compatible with conventional, existing monaural receiving apparatus. In one such proposal, the amplitude of an AM carrier is modulated to provide conventional monaural information which, of course, can be reproduced by existing AM radio receiving apparatus, and to phase modulate the AM carrier with stereophonic information, which phase modulations may be detected by receivers which are specially provided with suitable phase detector circuitry.
With this proposal, if monaural information is represented as L+R, wherein L represents left-channel information and R represents right-channel information, and if stereophonic information is represented as L-R, then an AM stereophonic signal S.sub.0 may be represented as: EQU S.sub.0 =(1+L+R) cos (.omega.t+.PHI.) (1)
wherein .PHI. is a function of the stereophonic information.
From equation (1), it is appreciated that the component (1+L+R) appears as amplitude modulations of the carrier, and the stereophonic component appears as phase modulations .PHI. of that carrier. Equation (1) may be rewritten as: ##EQU1## wherein: .omega. is the angular frequency of the signal carrier
P is a pilot signal (whose pilot frequency is in the range 5 to 25 Hz) ##EQU2## S.sub.A =(1+L+R) cos .omega.t-(L-R+P) sin .omega.t (5).
From equations (3), (4) and (5), it is seen that the AM stereophonic signal S.sub.0 has a carrier component cos .omega.t, which carrier component is amplitude-modulated by mono information (1+L+R), and also includes a stereo component which is represented by cos .PHI. in equation (3). From equation (1), it is seen that .PHI. represents the phase modulations of the signal carrier.
A conventional AM radio receiver serves to recover only the mono amplitude-modulated component of the AM stereophonic signal S.sub.0. The phase modulations of the received AM carrier are not detected and, hence, the stereo component cannot be demodulated. One type of AM stereo receiver that has been proposed for demodulating the AM stereophonic signal S.sub.0 operates by removing the cos .PHI. component of equation (3), resulting in the signal component S.sub.A represented in equation (5), and then detecting the amplitude modulations of the cos .omega.t component as well as the amplitude modulations of the sin .omega.t component. The information recovered by these detected amplitude modulations then are combined in a straightforward manner to recover the left-channel and right-channels signals L and R.
In an AM stereo receiver of the foregoing type, a phase-locked loop normally is provided in order to generate the various detecting and demodulating signals. However, some conditions may exist which would prevent the proper demodulation of the stereo information. For example, the phase-locked loop may not be phase-locked to the proper signal. Such an error condition in the operation of the phase-locked loop would defeat satisfactory stereo recovery. As another example, the signal intensity level of the received AM stereophonic signal may be so weak that the stereo information carried thereby cannot be recovered. It is desirable, under these circumstances, to operate the stereo receiver as a mono receiver so as to recover at least the monaural information which is carried by the received AM stereophonic signal.